Spacecraft army gears up to watch rare sun-diving comet

Once hailed as the “comet of the century”, ISON is now looking like a dud for casual skywatchers. But that won’t stop an army of spacecraft gathering to watch as the comet begins its dive toward the sun this week. This is a rare opportunity to glimpse a four-billion-year-old sample of the primordial solar system.

The preparations come just as NASA has pronounced dead its most famous comet-watching spacecraft, Deep Impact.

When Comet ISON was discovered last year, it was so bright that some thought it would outshine the moon. But it hasn’t brightened as much as predicted. Its early outbursts, originally thought to herald a huge comet, were instead carbon dioxide frost evaporating from a metre-thick coating it collected in the solar system’s frigid outer reaches.

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Those warning flares were a boon to astronomy.

“We have never discovered a comet that gets this close to the sun this far out before,” says Carl Hergenrother at the University of Arizona. The early warning gave astronomers enough time to plan a comprehensive observing campaign, which is rarely possible for sun-grazing comets.

Solar system sentinels

The first close-up of ISON will come on 1 October, when it flies within 10 million kilometres of the Red Planet. “Mars has the best seat in the solar system,” says Carey Lisse of the Johns Hopkins University Applied Physics Laboratory. The HiRISE telescope on NASA’s Mars Reconnaissance Orbiter will snap sharp pictures on a par with Hubble’s images. The European Space Agency’s Mars Express will also watch, alongside NASA’s Curiosity and Opportunity rovers.

Around the same time, the first balloon mission to study a new-found comet will gaze up from Earth. The Balloon Rapid Response for ISON mission will launch from New Mexico in late September to measure water vapour and carbon dioxide emitted from the comet. “No other spacecraft can make these measurements,” says Andy Cheng, also at the Johns Hopkins University Applied Physics Lab.

The Messenger spacecraft will spectate from its orbit around Mercury in October, and a suite of solar space-telescopes will track the comet’s passage closest to the sun on 28 November. Hubble and large ground-based telescopes will observe when the comet is further from the sun.

Unfortunately, NASA’s Deep Impact won’t join in the festivities. The craft, which made a close-up investigation of comet Tempel 1 in 2005, made early observations of ISON in 2012. Last month, NASA lost contact with it, and they officially declared it lost today.

Dusty fireworks

Still, the combined observations of ground and space-based telescopes will be enough to gain key insights into ISON’s origin and composition.

The real fireworks will happen as the comet begins to boil. When it reaches perihelion – the point in its orbit when it is closest to the sun – rocks and dust on the comet’s surface will vaporise, allowing astronomers to measure what it is made of.

This fiery trip will also test the comet’s strength.

Astronomers are debating whether fragile ISON will survive its encounter, but science and spectacle might both benefit from a crack-up, says Timothy Spahr, director of the Minor Planet Center at the Harvard-Smithsonian Center for Astrophysics. “What would be neat is if it fractured on the way to perihelion, and we got some really good spectra of a fresh comet,” he says.